Unlearning the Block Universe: How Relativity Challenges Our Perception of Time

[Lynch101]

That doesn't help because it doesn't tell me how to test whether it applies equally to past, present, and future events. Basically this looks to me like a way of dodging a question one doesn't have a good answer to.

You keep talking as if this is a viable alternative. It's not. As I've already explained, at a minimum, the events in the past light cone of your present event are fixed and certain, which would seem to mean they, at least, must "exist".

I also note that you keep switching terminology. First it was "real", now it's "exist", and in another post you used "form part of the structure of the Universe". None of those are really scientific terms; I've already explained why "real" isn't, and the explanation for "exist" or "form part of the structure of the Universe" would be similar. I suggested "fixed and certain" as a better alternative; see further comments below.

I've grouped these comments together because I think they all speak to the same point. I acknowledge that I keep changing terminology. That is an attempt to find a more suitable way of describing what the Block Universe says. That is what I am trying to do first, simply establish what the BU says.

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I'm returning to the top of this reply just to mention that I have been replying to your proposal of the term "fixed and certain" below. It certainly a preferable term to that of "real" but I think it needs some further clarification before it can be used in this context. Perhaps, more accurately, I need further clarification before I will be able to start using it in this context.
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What does the fox BU say?
You are obviously well familiar with the Block Universe, given your writings on it; and hopefully I'm not confusing you with another poster, but I think you've said that the Block Universe is a possible interpretation of relativity. But , you argue that it is not an absolute necessity, as often tends to be presented. Am I correct in that?

To start, we're just trying to establish what the familiar picture of the Block Universe says - aside from the claim that it is a necessary conclusion. Prior to that, we just need to be on the same page about what the BU says about past and future events. You might be able to state that more rigorously than I can but the BU definitely says something about past, present, and future events. It says something about past and future events that makes it different to a universe that is based only on a global "now".

Can we say past and future events have the same status?/ontology?/constitution?/[insert the most precise possible term here] as present events.

Ontology
Instead can we talk about the ontology of events that constitute a world line. The BU says that all points on a world line are ontologically equal. No single event is preferred over another. The observations we make in our scientific experiments might correspond to single events on our world line but that doesn't single it out as special.

To give a real world example, again, of our 10/30/50th birthdays. If now is our 30th birthday then we consider our 10th birthday to be in the past and our 50th birthday to be in the future. A universe with a global present comprises only the event of our 30th birthday, only a single event on our world line. The Block Universe, however comprises all of those events 10/30/50th birthdays. None of the events is singled out over the others.Dodging the question
I don't want to get bogged down on this particular point because I know how endless these philosophical rabbit holes can be, so I am keen to avoid such a debate. I think it's more important to just get on the same page as to what the BU says about past, present, and future events. I don't want to dodge the point above, however.

The way it is used here, the term "real" isn't a set of criteria which something must fulfil, it is instead simply a label that we can try to apply in our discussion. If we apply the label "real" to present events, say our 30th birthday, then the Block Universe says that label applies equally to past events, our 10th birthday, as well as future events, such as our 50th birthday. We don't need to know the true nature of reality to use the term in this way, instead, if we say that our 30th birthday is real, then we must say the same thing about our 10th and 50th birthdays.

We could use a different label, but "real" has the benefits of our preconceptions of what it means. It is a double edged sword however, for the reason you raise above, but any other term would probably cause more confusion.

No, it doesn't. Newtonian physics is perfectly consistent with viewing the past as fixed and certain. It is also consistent, as has already been pointed out, with a "block universe" view in which the future is fixed and certain as well as the present and past (this works because Newtonian physics is deterministic). So it seems like you were given an incorrect view of Newtonian physics as well as relativity.

Thank you Peter, yes. My apologies, when I refer to the Newtonian picture I am referring to the conception of it that is best juxtaposed with the Block Universe to offer a contrast. From here on, if I say the Newtonian picture, I am referring to the conception that is based on a global "now" not on a Newtonian Block Universe.

That what happens at them is fixed and will not change. Or, to put it another way, if we consider all possible 4-d spacetime models that could be realized, given what you know at your present event, all of them will have the same set of events (things that happen) in the past light cone of your present event. But not all of them will have the same set of events (things that happen) outside of the past light cone of your present event.

As you have mentioned above, the term "fixed and certain" can be applied equally to a Newtonian universe, either with a single global "now" or a Newtonian block universe, to a relativistic block universe, or in another way that isn't equivalent to the BU.

These represent very different pictures of the Universe, so the idea of "fixed and certain" would need some additional clarification. You mentioned above that the BU considers future events to be "fixed and certain", so, according to your application, we only consider events in the past light cone to be fixed and certain, it sounds very much like the conceptualisation usually referred to as "the growing block universe".

You say

My proposal is consistent with this view, as far as I can see, but my proposal has the advantage, from a scientific point of view, of not making claims that are not scientific.

As you have said, your proposal is consistent with all the different views, but all of the different views present very different pictures of the Universe, so I think it is necessary to clarify what we are referring to when we use it.

Is it intended to be more of a catch all phrase?

You say

As I've pointed out, block universe proponents ignore this obvious fact because they look at models that they have constructed, in which they declare by fiat what events happen everywhere in the 4-d spacetime of their model. But the real world doesn't work like that. You can't dictate by fiat what happens outside your past light cone. You can try to predict what will happen outside your past light cone, but those predictions can never be perfect, because you don't have sufficient data in your past light cone to determine for certain what will happen at any event outside your past light cone.

Note that this is true even if the fundamental physical laws are deterministic; even in a deterministic system, in order to have sufficient data to fix all events everywhere in the spacetime, you need to have initial data on an entire spacelike 3-surface. But no past light cone contains such data, and nobody ever has or ever will have such data. I discuss this in my Insights article (and IIRC there was more discussion of it in the comment thread on it).

Absolutely agree on this point. We cannot define what happens outside our past light cone.

I don't think this affects our conclusion though. We can talk strictly about the events that constitute our own world line and reason from there.
You mentioned

The fix for this is simple: don't try to learn actual science from pop science sources. Brian Greene in particular is a frequent offender--if I had a dollar for every PF thread where we've had to correct someone's misconceptions based on one of his pop science books or shows or videos, I'd be retired now.

If I could go back and choose again (and if free will really exists) I would choose to study physics. Unfortunately, my past is fixed and certain so that isn't an option.
I am grateful for pop-science though because it has allowed me to at least engage with physics, where otherwise I would not have. My understanding of physics is certainly better because of pop-science material, because without it, it would be non-existent. Even if it means the arduous task of unlearning some things, but I feel like it's at least given me a foundation from which to start.You say

This argument is simply wrong. It's not even as good as the argument I refuted in my Insights article, which at least made some pretense of using concepts specifically from relativity. This argument could just as well be made for Newtonian physics, and is just as wrong when applied to that. The obvious flaw is the smuggling in of "past and future events" as though they had to go together, when they obviously don't.

I'm not sure what you mean by smuggling in "past and future" events (with emphasis on and). Are you implying that relativity necessitates a block universe which comprises past and present events, but not necessarily future events?

Or are you saying that neither past nor future events can be "smuggled in"?I'm not actually sure how I'm smuggling anything in. I'm simply juxtaposing two alternatives and saying if not this, then that.

 

[Lynch101]

If you do want to avoid constraining "real", that includes not specifying whether it does or does not apply equally to events in the "past" and "future", whatever those terms might mean.

If now is your 30th birthday, then your 10th birthday would be a "past event", while your 50th birthday would be a "future event".

 

[Lynch101]

Not quite. I am proposing that you may believe there exists a unique global now, but you will never be able to find out what it is except that your local present is part of it. Such a belief in something you can't verify isn't really different from belief in the block universe. Both beliefs are compatible with relativity, neither can be verified.

That's true, but we can specify that either a global now exists, or it doesn't. If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

 

[Lynch101]

Here it is then: The events that are happening now are the ones whose time coordinate is the same as the time coordinate of the event “I just said ‘now’”. That can be extended into the more general notion of “at the same time”: all events that have the same time coordinate are said to have happened at the same time. An important special case is when an event has the same time coordinate as the event “my wristwatch reads X”; I will say that the event happened at time X.

These terms ("now", "happened when", ...) tell us more about how we assign time coordinates to events than the relationships between events.

Thanks Nugatory. That's similar to how Einstein defines simultaneity in his paper on relativity, isn't it?

I was thinking more along the lines of trying to define it without reference to clocks or simultaneity, but it might not be possible to do that rigorously.

 

[Lynch101]

Of course. If you compare presentism and eternalism you will clearly have a different conceptualization of time.

The point is that both conceptualizations are compatible with Newtonian physics and both conceptualizations are compatible with relativistic physics. Neither conceptualization is forced on us by the physics.

By “the physics” I mean the experimental evidence and the mathematical framework used to make experimental predictions. It seems like you may be adding some philosophy to the physics in what you understand relativity to be. So you are already assuming a specific conceptualization of time which is not part of “the physics”.

So LET shares all of the mathematical framework and all of the experimental predictions of the block universe. The physics of both is the physics of SR. They differ in their conceptualization of time. This shows that the conceptualization of time is not part of the physics

Presentism would necessitate absolute simultaneity though, which the relativity of simultaneity tells us isn't correct. So presentism can't be compatible with Einstein's relativity.

 

[Dale]

From here on, if I say the Newtonian picture, I am referring to the conception that is based on a global "now" not on a Newtonian Block Universe.

I would prefer if you not do that.

The correct term is “presentism”. If you use the term “Newtonian” to refer to presentism then the terminology makes the whole discussion more cumbersome. We know that relativistic picture of physics is different from the Newtonian picture of physics in many ways. So if you label “presentism” as Newtonian then it becomes difficult to disentangle the presentism philosophy from the Newtonian physics, the mathematical framework and experimental predictions. The philosophy and the physics are separate things.

Please use the correct philosophical term “presentism”.

If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

“The physics” (experimental predictions and mathematical framework) of relativity is perfectly compatible with an undetectable global now, just as it is compatible with an undetectable aether frame.

 

[PAllen]

That's true, but we can specify that either a global now exists, or it doesn't. If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

No, it would not. The relativity of simultaneity is what would make a global past / future surface undetectable. It doesn't prove its nonexistence.

 

[PeterDonis]

That is what I am trying to do first, simply establish what the BU says.

Then, instead of trying to make up your own terminology, why don't you read what the proponents of the BU interpretation actually say? For example, you could read the very statements I quoted in my Insights article, or the ones made in the references I gave there.

I have been considering your proposal of the term "fixed and certain". It certainly a preferable term to that of "real" but I think it needs some further clarification before it can be used in this context. Perhaps, more accurately, I need further clarification before I will be able to start using it in this context.

If you will read my Insights article, you will see that I chose that term because Roger Penrose, in the quote from The Emperor's New Mind that I gave, uses "certain" and "fixed" in his own description of the argument that I refute in the article.

I really wonder whether you have actually read my Insights article with proper attention, since you keep bringing up issues that I explicitly addressed there, precisely in order to try and forestall the kind of rehashing of them that we seem to be doing here.

I think you've said that the Block Universe is a possible interpretation of relativity. But , you argue that it is not an absolute necessity, as often tends to be presented. Am I correct in that?

Yes.

The way it is used here, the term "real" isn't a set of criteria which something must fulfil, it is instead simply a label that we can try to apply in our discussion.

And a useless one, as far as science is concerned, since it has no experimental consequences. Science is not concerned with applying labels to things that have no experimental consequences. Science is concerned with things we can actually test by experiment.

Of course people like to label things in ways that science doesn't address; that's fine. But it's not science. And to the extent the BU interpretation involves such labels, it's not science either. It's philosophy, or metaphysics, or whatever you want to call it other than "science", but it isn't science.

We could use a different label, but "real" has the benefits of our preconceptions of what it means.

What you see as a benefit, I see as a hindrance. Preconceptions are not useful if they lead you to make claims that are not valid, such as BU proponents claiming that BU is required by relativity.

any other term would probably cause more confusion.

"Fixed and certain", as I noted above, was the term used by Penrose, and it seems pretty clear what he means by it. It certainly works a lot better in his presentation of the argument than "real" would have.

These represent very different pictures of the Universe, so the idea of "fixed and certain" would need some additional clarification.

These represent very different pictures of the part of the Universe we don't know about yet, i.e., the part of the Universe that we haven't yet observed because no information from that part has reached us, because of the finite speed of light. But all of these very different pictures of the Universe share the belief that the things we already know about won't change. What you ate for breakfast yesterday isn't going to change depending on what you observe tomorrow. Where you were born, where your parents went to school, etc., aren't going to change depending on whether or not it rains tomorrow. And so on. All of these very different views of the Universe agree on simple mundane things like that.

That common property that all of these very different pictures of the Universe agree that the things we already know about have, is what I mean by "fixed and certain". The question is whether any other parts of the Universe, besides the part we already know about, have this property. The BU view is the view that all of the events everywhere in 4-d spacetime have this property. The argument I refute in the Insights article claims that that view is required by relativity. It's not.

according to your application, we only consider events in the past light cone to be fixed and certain, it sounds very much like the conceptualisation usually referred to as "the growing block universe".

I've already said that my proposed view is consistent with the "growing block universe" view. But your calling it the "growing block universe" view does not mean it's the same as the block universe view I discuss in my Insights article. It's not. The term "growing block universe" is thus a very unfortunate term to describe that view.

We can talk strictly about the events that constitute our own world line

Not all of them, since only the events on your worldline that are either your present event, or to the past of your present event, are in your past light cone. Events on your worldline to the future of your present event, such as you eating breakfast tomorrow, are not.

I'm not sure what you mean by smuggling in "past and future" events (with emphasis on and).

You are talking as if past events and future events must have the same "ontological status" (or whatever term you want to use). That's obviously false. There is nothing at all inconsistent about saying that past events are fixed and certain but future events are not. So there is nothing that requires past and future events to always be treated on the same footing.

But the argument you were quoting assumes that that is required: it claims that the only alternative to a block universe view, i.e., to having both past and future be "as real as" (or whatever term you want to use) the present, is to have only the present be "real" (or whatever term you want to use). If it's not required to treat the past and future on exactly the same footing, then the obvious other alternative is to treat the present and the past as "real" (or whatever term you want to use), but not the future. (And with the proper relativistic definition of "present" and "past", i.e., the "present" is your present event, and the "past" is its past light cone, that is exactly why my proposed alternative view does.)

The fact that this obvious alternative is not even mentioned is why I say the argument you were quoting here is not even as good as the one I refuted in my Insights article.

Are you implying that relativity necessitates a block universe which comprises past and present events, but not necessarily future events?

Not quite, no. First, "present" is only one event--your present event. Second, "past" is just your past light cone, which not everyone is clear about. Third, "future" is not all of the rest of spacetime; it's just your future light cone. There is also the region I called "elsewhere" in my Insights article (another useful term from Roger Penrose, who uses it in The Emperor's New Mind and, IIRC, other books as well), all of the events that are spacelike separated from your present event.

I'm simply juxtaposing two alternatives and saying if not this, then that.

Which is invalid reasoning if there are any other alternatives that you have not considered. Which there are in this case. See above. This logical fallacy is so common that it has a name: "False dichotomy".

 

[Nugatory]

Thanks Nugatory. That's similar to how Einstein defines simultaneity in his paper on relativity, isn't it?

It is the definition that he used, but it’s not his definition - it‘s the only definition that has ever been used (although usually not so explicitly) by anyone. Einstein’s contribution was to show the limitations of this generally accepted definition of “now”.

 

[Lynch101]

I would prefer if you not do that.

The correct term is “presentism”. If you use the term “Newtonian” to refer to presentism then the terminology makes the whole discussion more cumbersome. We know that relativistic picture of physics is different from the Newtonian picture of physics in many ways. So if you label “presentism” as Newtonian then it becomes difficult to disentangle the presentism philosophy from the Newtonian physics, the mathematical framework and experimental predictions. The philosophy and the physics are separate things.

Please use the correct philosophical term “presentism”.

Thank you Dale. I know that philosophy is off topic here, so I wanted to ground it in the context of scientific theory. Apologies if this caused confusion.

“The physics” (experimental predictions and mathematical framework) of relativity is perfectly compatible with an undetectable global now, just as it is compatible with an undetectable aether frame.

Maybe I'm interpreting this incorrectly then, but it would seem to suggest that relativity is compatible with absolute simultaneity, because presentism would necessitate absolute simultaneity. Is it compatible with absolute [undetectable] simultaneity?

 

[Lynch101]

No, it would not. The relativity of simultaneity is what would make a global past / future surface undetectable. It doesn't prove its nonexistence.

Ah I see. I was told that the relativity of simultaneity ruled out absolute simultaneity.

Or is it compatible on the basis of how simultaneity is defined, as opposed to on the basis of an underlying metaphysics?

 

[Dale]

it would seem to suggest that relativity is compatible with absolute simultaneity, because presentism would necessitate absolute simultaneity. Is it compatible with absolute [undetectable] simultaneity?

Yes. It is compatible with LET and from there presentism is easy to see.

I know that philosophy is off topic here, so I wanted to ground it in the context of scientific theory. Apologies if this caused confusion.

This is inherently a purely philosophical discussion. Using incorrect terminology doesn’t suddenly ground the discussion in science. “Block universe” is a standard synonym for “eternalism”. But “Newtonian view” is not a standard synonym for “presentism”, and I object to its use.

 

[PAllen]

Ah I see. I was told that the relativity of simultaneity ruled out absolute simultaneity.

Or is it compatible on the basis of how simultaneity is defined, as opposed to on the basis of an underlying metaphysics?

Not sure why we keep talking in circles. Try this: if Newtonian physics were true, there would be a procedure that could actually be performed to synchronize separated clocks such that any pair of clocks synchronized in one frame would be synchronized when compared to relatively moving clocks synchronized using the same procedure by someone comoving with those clocks. This is what absolute simultaneity means.

In relativity, this is false - it cannot be done.

None of this determines what is the boundary between what is fixed and certain versus not is. A block universe interpretation of either Newtonian physics or relativity simply posits that all that ever will be is as fixed as all that ever was. In neither theory is it possible to test this belief. Similarly either theory is compatible with the idea that there is a unique boundary defining a global now that separates what is fixed and certain from what is not. In neither theory is it possible to experimentally address this belief. The only difference is that in Newtonian physics it would be natural to identify this boundary with the absolutely simultaneous times as described above. However, you would still have no way to prove this over a block universe belief.

 

[Lynch101]

I just want to start this post by saying that I think I see the point you were trying to make, that we can't assume that the universe comprises events which are in our future light cone, we can't say that those events are fixed and certain. There are still one or two things I think I'm not be clear on. I'll reply below, but I just wanted to put this in at the start.

I may have confused the issue by using the term Block Universe because I have also been saying if past and/or future events, so I haven't necessarily been making an assumption about future events, although my use of the term Block Universe has clouded the issue.

Then, instead of trying to make up your own terminology, why don't you read what the proponents of the BU interpretation actually say? For example, you could read the very statements I quoted in my Insights article, or the ones made in the references I gave there.

If you will read my Insights article, you will see that I chose that term because Roger Penrose, in the quote from The Emperor's New Mind that I gave, uses "certain" and "fixed" in his own description of the argument that I refute in the article.

I really wonder whether you have actually read my Insights article with proper attention, since you keep bringing up issues that I explicitly addressed there, precisely in order to try and forestall the kind of rehashing of them that we seem to be doing here.

"Fixed and certain", as I noted above, was the term used by Penrose, and it seems pretty clear what he means by it. It certainly works a lot better in his presentation of the argument than "real" would have.

I wasn't familiar with the terminology you used in the article so it didn't stick in my mind. I understood what was meant as I read it, but the terminology I have become accustomed to is speaking about past and future as "real", or about the ontological nature of events on a world line. This seemed to be a major stumbling block, so I was trying to throw terms out there to see if I could illustrate the point I was trying to make, so that we would be on the same page. I was unfamiliar with your use of "fixed and certain", so I wasn't confident of using them in such a way as to convey my own understanding.

These represent very different pictures of the part of the Universe we don't know about yet, i.e., the part of the Universe that we haven't yet observed because no information from that part has reached us, because of the finite speed of light. But all of these very different pictures of the Universe share the belief that the things we already know about won't change. What you ate for breakfast yesterday isn't going to change depending on what you observe tomorrow. Where you were born, where your parents went to school, etc., aren't going to change depending on whether or not it rains tomorrow. And so on. All of these very different views of the Universe agree on simple mundane things like that.

That common property that all of these very different pictures of the Universe agree that the things we already know about have, is what I mean by "fixed and certain". The question is whether any other parts of the Universe, besides the part we already know about, have this property. The BU view is the view that all of the events everywhere in 4-d spacetime have this property. The argument I refute in the Insights article claims that that view is required by relativity. It's not.

If I understand correctly, your argument refutes the idea that events in our future light cone are fixed and certain, but allows for differing views about events in our past light cone.

It seems to be compatible with a presentist universe, but also with what is often colloquially referred to as the growing block, as you mention below.

I've already said that my proposed view is consistent with the "growing block universe" view. But your calling it the "growing block universe" view does not mean it's the same as the block universe view I discuss in my Insights article. It's not. The term "growing block universe" is thus a very unfortunate term to describe that view.

I understand, yes, the growing block is a different idea form the fully formed block.

This would mean that the "fixed and certain" definition applies to either a presentist or "growing block" type universe, am I correct in that?

Not all of them, since only the events on your worldline that are either your present event, or to the past of your present event, are in your past light cone. Events on your worldline to the future of your present event, such as you eating breakfast tomorrow, are not.

I understand your point now, I believe. This would mean that the Universe comprises either our present event, or our past and present events, but not necessarily our future events.

I was under the impression that when we add in a relatively moving observer, and their calculations about when events must have happened, based on when light from an event reaches them, leads us to the idea that the universe must also comprise events in our future light cone.

As in the example of the relatively moving observers on the train and platform, and the lightning strikes.

You are talking as if past events and future events must have the same "ontological status" (or whatever term you want to use). That's obviously false. There is nothing at all inconsistent about saying that past events are fixed and certain but future events are not. So there is nothing that requires past and future events to always be treated on the same footing.

I see how your criterion about events in our past light cone necessitates this. But my understanding was that, when we have two relatively moving observers, one of them either observes or calculates that an event which is part of their present (the flash of light to the front of the train) is in the future light cone of the other observer. Similarly, the observer on the platform can calculate, after the fact, that an event in their future light cone must have already happened when the other observer passed them.

Even if the observer on the platform can't say, in the present, that the event in their future light cone is fixed and certain, their calculations lead them to the conclusion that the future event must have been fixed and certain.

If I have understood the explanation correctly.

But the argument you were quoting assumes that that is required: it claims that the only alternative to a block universe view, i.e., to having both past and future be "as real as" (or whatever term you want to use) the present, is to have only the present be "real" (or whatever term you want to use). If it's not required to treat the past and future on exactly the same footing, then the obvious other alternative is to treat the present and the past as "real" (or whatever term you want to use), but not the future. (And with the proper relativistic definition of "present" and "past", i.e., the "present" is your present event, and the "past" is its past light cone, that is exactly why my proposed alternative view does.)

The fact that this obvious alternative is not even mentioned is why I say the argument you were quoting here is not even as good as the one I refuted in my Insights article.

I see the assumption I was making now, but, if my understanding of what was explained to me is correct, it means that the picture of the Universe that relativity allows us to calculate, isn't just dependent on what is in our own past light cone. We can calculate what a relatively moving observer would observe and when events happen in the relatively moving frame, cross reference with the location of the observer in our frame, and then make further deductions about events that would have been in our future light cone.

This is all calculated after the fact, but it allows us to build a more complete picture...I think.

Not quite, no. First, "present" is only one event--your present event. Second, "past" is just your past light cone, which not everyone is clear about. Third, "future" is not all of the rest of spacetime; it's just your future light cone. There is also the region I called "elsewhere" in my Insights article (another useful term from Roger Penrose, who uses it in The Emperor's New Mind and, IIRC, other books as well), all of the events that are spacelike separated from your present event.

OK, maybe I haven't fully understood your point then. Am I right in saying that we can't consider events in our future light cone as fixed and certain, at the very least?

If relativity doesn't necessitate that the Universe comprises

Is the above description talking about one single observer and what they can say is fixed and certain at any given moment?

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer. So, although they cannot say in advance that events in their future light cone are fixed and certain, they can calculate afterwards that such events must have been fixed and certain.

I might be misinterpreting that, but that is how I have understood it. It might be easier to explain by way of a thought experiment.

Which is invalid reasoning if there are any other alternatives that you have not considered. Which there are in this case. See above. This logical fallacy is so common that it has a name: "False dichotomy".

I have been talking about past and/or future events, so I haven't really been making an assumption about future events, although I can see how the Block Universe is the idea that past and future events "exist".

 

[PeterDonis]

If I understand correctly, your argument refutes the idea that events in our future light cone are fixed and certain, but allows for differing views about events in our past light cone.

No. I have said repeatedly that events in our past light cone are fixed and certain on all views (although I did not previously mention presentism, which is a possible exception--but see below). Certainly events in our past light cone are fixed and certain on the alternative view I proposed in the article. The question is whether any events other than our present event and its past light cone are fixed and certain.

In any case, my argument did not refute the claim that events in our future light cone are fixed and certain. It only refuted a particular argument that purports to claim that relativity requires events in our future light cone to be fixed and certain. I have explained this several times now.

It seems to be compatible with a presentist universe

It depends on what you mean by "presentism", and discussions of that view are too contaminated with vague terms like "real" for me to know what they are really saying. Also, the term "present" is ambiguous; on the alternative view I gave in the article, it means "your present event", but on many "presentist" views, it seems to mean "some spacelike 3-surface that contains your present event".

This would mean that the "fixed and certain" definition applies to either a presentist or "growing block" type universe, am I correct in that?

I am using the term "fixed and certain" as a general term that can be used by any viewpoint; what each viewpoint does is specify exactly which events are fixed and certain.

This would mean that the Universe comprises either our present event, or our past and present events, but not necessarily our future events.

This describes two possible viewpoints, yes.

I was under the impression that when we add in a relatively moving observer, and their calculations about when events must have happened, based on when light from an event reaches them, leads us to the idea that the universe must also comprise events in our future light cone.

Sorry to shout, but THIS IS EXACTLY THE ARGUMENT I REFUTED IN MY ARTICLE. Have you really read it? Really? Are you sure you don't need to go back and read it again?

Am I right in saying that we can't consider events in our future light cone as fixed and certain, at the very least?

No. You have repeatedly failed to understand what I actually said in the article.

Here is what I said in the article: Relativity does not require all events in 4-d spacetime to be fixed and certain.

Here are some things I explicitly did not say in the article:

I did not say that events in our future light cone are definitely not fixed and certain. (I only said relativity does not require that they are.)

I did not say that only our present event is fixed and certain. Nor did I say that it isn't.

I did not say that only some spacelike 3-surface containing our present event is fixed and certain. Nor did I say that it isn't. (I did propose an alternative that would say it isn't, except for our present event itself, but that's a different thing.)

I did not even say that relativity requires events in our past light cone to be fixed and certain. (I only said that this view is an obvious alternative that (a) makes sense, and (b) was not even proposed or considered by all the many people who have argued about the block universe.)

I hope this helps.

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer.

No, that's not what they can calculate after the fact. No calculation can tell you "when" some distant event happened according to some observer. That is something you assign by choosing coordinates. It has no physical meaning whatsoever.

What observers can calculate after the fact are the causal relationships between events: which events are in which other events' past or future light cones, which events are spacelike separated from which other events. Those things are invariants and have physical meaning. But in order to do those calculations, all of the events the calculations apply to must already be in the past light cone of the person calculating them. Which means nobody ever has to treat any events other than those that are in their past light cone as fixed and certain in order to do calculations. Which I pointed out in my article.

 

[Nugatory]

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer.

That cannot happen.

It can happen that an event in the past light cone of A ("A considers it to have happened") is in neither the past nor the future light cone of B ("Anything B says about whether it has happened or not depends on B's completely arbitrary choice of convention for assigning time coordinates to events outside the the light cones") or vice versa. And note that even that cannot happen when they're passing one another; when they're both at the same point in space they have teh same past and future lightcones.

 

[Dale]

It depends on what you mean by "presentism", and discussions of that view are too contaminated with vague terms like "real" for me to know what they are really saying.

I encouraged him to use the term “presentism” rather than “Newtonian”. As you say, the whole concept of presentism is focused on identifying what is “real”. Since that is a philosophical term of art it makes it clear that this discussion is a philosophical rather than a scientific discussion. I don’t think we should discourage the use of the correct terminology here.

I like “fixed and certain” also, but I am not sure how it relates to the philosophical concept of “real”. I suspect that a presentist would accept the future and past as “fixed and certain” under a deterministic theory even though he would not accept them as “real”.

 

[PeterDonis]

I like “fixed and certain” also, but I am not sure how it relates to the philosophical concept of “real”.

I'm not either. By suggesting that term, I was attempting to focus discussion on something that seemed less prone to vagueness and imprecision than "real". I'm not sure whether that attempt has actually worked.

 

[Lynch101]

No. I have said repeatedly that events in our past light cone are fixed and certain on all views (although I did not previously mention presentism, which is a possible exception--but see below). Certainly events in our past light cone are fixed and certain on the alternative view I proposed in the article. The question is whether any events other than our present event and its past light cone are fixed and certain.

In any case, my argument did not refute the claim that events in our future light cone are fixed and certain. It only refuted a particular argument that purports to claim that relativity requires events in our future light cone to be fixed and certain. I have explained this several times now.
...
Here is what I said in the article: Relativity does not require all events in 4-d spacetime to be fixed and certain.

Here are some things I explicitly did not say in the article:

I did not say that events in our future light cone are definitely not fixed and certain. (I only said relativity does not require that they are.)

I did not say that only our present event is fixed and certain. Nor did I say that it isn't.

I did not say that only some spacelike 3-surface containing our present event is fixed and certain. Nor did I say that it isn't. (I did propose an alternative that would say it isn't, except for our present event itself, but that's a different thing.)

I did not even say that relativity requires events in our past light cone to be fixed and certain. (I only said that this view is an obvious alternative that (a) makes sense, and (b) was not even proposed or considered by all the many people who have argued about the block universe.)

I hope this helps.

Thank you Peter, I appreciate your patience in all of this. I understand that it must get frustrating to have fielded such questions as these previously, gone to the trouble of writing an article that addresses these questions, and then have someone come along and raise the same old questions again.

I have read your article, but I didn't fully comprehend it, as is evident. The purpose of this thread is, in essence, an exercise in trying to understand it. I'm starting from a position where I have been taught that the BU = relativity, so I am only really familiar with arguments in favour of the BU. As with moving from a Newtonian picture to a relativistic one, I found that putting forward my own understanding and then having the errors and assumptions pointed out, helped me to understand, in a conceptual [but limited] way, what relativity says - apparently with a few added assumptions.

I think we were talking past each other a little up until this point. In the OP I tried to avoid the assumption that past and future events are necessitated by saying past and/or future. I think the problem is that I was using this interchangeably with the Block Universe which says that past and future events are fixed and certain.

Am I correct in saying that relativity allows for the following possibilities:

• [Whole] Block Universe
• Past Block Universe - colloquially the "growing block"
• Future Block - we might call this a "shrinking block".
• Presentist
• An alternative based on events in the past light cone are fixed and certain.

I'm not actually familiar with any advocates for a "shrinking block", and I'm not sure it is genuinely a possibility, but I've included it here just for [possible] completeness. Is there another alternative, as per the 5th option there? Is this what you suggest in the article? I'm not sure I fully understand it, if so.

If the above possibilities are correct, and if they are taken together with your statement in the other thread:

This also means there is no global concept of "now" in relativity.

Would this mean that we can rule out presentism?

If we rule out presentism, then wouldn't we be left with a universe which comprises past and/or future events, so either:
past, present, and future
past and present
present and future
Some alternative which includes some past events, but not all?

The confusion being that I was conflating the 2nd and 3rd options with "the Block Universe", which is the first option.Hopefully, I've got that much correct. I still don't fully understand why the full block isn't necessitated. I think I can see the destination of your argument but I'm not yet, fully sure how to get there. Some of the further points below are key to that I think.

It depends on what you mean by "presentism", and discussions of that view are too contaminated with vague terms like "real" for me to know what they are really saying. Also, the term "present" is ambiguous; on the alternative view I gave in the article, it means "your present event", but on many "presentist" views, it seems to mean "some spacelike 3-surface that contains your present event".

I appreciate the difficulty with such terms. If I were better versed I might be able to define it in more rigorous terms, instead I am left to try and describe around it, in the hope that I communicate what I mean.

I think of presentism in the context of Newtonian physics, the global/Universal "now" which would necessitate absolute simultaneity. Sometimes its helpful to think of this juxtaposed with the Block Universe, the growing block, and the shrinking block.

I'm not entirely sure how to define it rigorously.

I am using the term "fixed and certain" as a general term that can be used by any viewpoint; what each viewpoint does is specify exactly which events are fixed and certain.

I think I get you. I think I struggle a little with defining "fixed and certain" as being those events that are in the past light cone of an observer and then applying this to the future light cone of the observer, as the block universe does. But maybe, we can just think that all events in a block universe are in the past light cone of "the end of the universe"?

If the term "fixed and certain" can be applied to all of the different models, and each of the models describes a different picture of the Universe, do we need another term for describing what those different models say.

"Fixed and certain" seems to tell us what we can definitively say about events in the past light cone, but it doesn't distinguish between a presentist interpretation or the BU interpretation, or btween the BU interpretation and a growing block interpretation. Each of these says something somewhat different about past and future events.

I have started using the term comprise so as to avoid the word "real".

Sorry to shout, but THIS IS EXACTLY THE ARGUMENT I REFUTED IN MY ARTICLE. Have you really read it? Really? Are you sure you don't need to go back and read it again?

No. You have repeatedly failed to understand what I actually said in the article.

No worries, I get that this must be incredibly frustrating. That genuinely isn't my intention. I have gone back and read the article again and I think I understood a bit more this time, but I'm still not certain I fully grasp it. As I mention above, I think I can see the destination, I just haven't progressed through the route to get there. This thread is intended to be the means of getting there.

No, that's not what they can calculate after the fact. No calculation can tell you "when" some distant event happened according to some observer. That is something you assign by choosing coordinates. It has no physical meaning whatsoever.

What observers can calculate after the fact are the causal relationships between events: which events are in which other events' past or future light cones, which events are spacelike separated from which other events. Those things are invariants and have physical meaning. But in order to do those calculations, all of the events the calculations apply to must already be in the past light cone of the person calculating them. Which means nobody ever has to treat any events other than those that are in their past light cone as fixed and certain in order to do calculations. Which I pointed out in my article.

This is the part where the gap in my understanding lies. I will try to outline how I have understood the explanation and maybe you can identify where I am going wrong. Again, I can see the destination, but I was lead to believe something else.

I find it helpful to talk in terms of thought experiments because it helps to make things a little less abstract.

=============================================================================================
Returning to this point after having written the below. It is written in a very "matter of fact" tone, but that is just because that is how it flowed. I figured it would be easier to come back and point this out, rather than try to go through the whole thing and try to edit the language to make it less so. As I say, this is just how I have understood the arguments that were put to me, so I am very much open to correction.
==============================================================================================If we think about Albert Einstein and Hendrick Lorentz moving relative to each other. Albert is on the platform and Hendrick is on the train. Both know the theory of relativity inside out. Albert is located at point M, which is midway between two light sources L(eft) & R(ight), which are at rest relative to him. As Hendrick whizzes along the track, in the direction of L to R, he passes Albert at point M. A few moments later, Albert observes two simultaneous flashes of light coming from each light source. That is, photons from both L and R arrive to his location simultaneously. Hendrick, on the train, observes the light from R first and the light from L second.

Afterwards, both retire to their respective studies to perform their calculations and see if they can arrive at any additional information. Both can perform Lorentz transformations to build a picture of things from the relatively moving reference frame.

Based on the information from his own reference frame, Albert calculates that the light emission events from L and R happened prior to the photons arriving to his location. All of those events are now in his past light cone, but he can calculate that those light emission events must have occurred earlier, in order to arrive to his location when they did. That is, the emission events must have been fixed and certain prior to the photons arriving to his location and being in his past light cone. At the time, he could not know this, but afterwards he can calculate that it must have been so. The reason being, photons can only be emitted from events which have happened or are fixed and certain. Events which are not fixed and certain, cannot emit photons.

If "fixed and certain" doesn't work in this context, then I will need to find another term to describe what it is that I am saying.

This doesn't tell us anything particularly interesting. This would have been the case in the Newtonian picture also. It is when Albert performs the Lorentz transformation that we acquire additional information. One additional point that Albert notes about the emission events form L and R, is that the events must have coincided with the moment when Hendrick was at point M. This is important, because it gives us a non-clock related reference point.

Albert's Lorentz Transformation
Albert performs a Lorentz transformation to get the picture according to Hendrick's reference frame. As he expected, he notes that the light from the two light sources did not arrive to Hendrick simultaneously, instead, light from R arrived first and light from L arrived second. Again, nothing surprising here. Where we get the additional information, is when Albert calculates when the emission events must have happened according to Hendrick.

According to Hendricks frame of reference, the emission events were not simultaneous, not just because the light didn't reach him simultaneously but due to the relativity of simultaneity. According to Hendrick's frame the emission event from R happened before the event from L. The additional information that Albert gets from this is the location of Hendrick when these events happened.

According to Albert's frame, the emission events happened at the precise moment that Hendrick was located at point M. However, according to Hendrick's frame, the emission event from R must have happened prior to his arrival at M. The same is true for Henry as it is for Albert, photons can only be emitted from events that have happened or events which are fixed and certain. Events which are not fixed and certain, or have not happened can not emit photons.

This means, Albert must agree that prior to Hendrick's arrival at point, the emission event from R must have been fixed and certain, otherwise Hendrick wouldn't have observed the photon where he did (in his own reference frame) and his theory of relativity would be inaccurate.

This is what leads to the conclusion that the universe comprises future events because the emission event from R was in Albert's future in the moments prior to Hendrick's arrival at point M, but it must have been fixed and certain in order to emit a photon.

 

[Lynch101]

Yes. It is compatible with LET and from there presentism is easy to see.

Ah, I see. I am familiar with the notion of LET incorporating presentism but I had come to understand the relativity of simultaneity as being antithetical to absolute simultaneity i.e. that one precluded the other.

This is inherently a purely philosophical discussion. Using incorrect terminology doesn’t suddenly ground the discussion in science. “Block universe” is a standard synonym for “eternalism”. But “Newtonian view” is not a standard synonym for “presentism”, and I object to its use.

Newtonian physics wouldn't be synonymous with presentism, but presentism would represent a fairly standard interpretation of Newtonian physics, I would have thought. Although Newtonian physics is compatible with the concept of a block unvierse, I've never heard anyone advocate for such an interpretation.

 

[Lynch101]

That cannot happen.

It can happen that an event in the past light cone of A ("A considers it to have happened") is in neither the past nor the future light cone of B ("Anything B says about whether it has happened or not depends on B's completely arbitrary choice of convention for assigning time coordinates to events outside the the light cones") or vice versa. And note that even that cannot happen when they're passing one another; when they're both at the same point in space they have teh same past and future lightcones.

I hope you don't take umbridge with me linking you to my reply to Peter, just to save typing the whole thing again.

 

[PeterDonis]

Am I correct in saying that relativity allows for the following possibilities

Yes, all of those possibilities are allowed by relativity (though, as you note, there don't seem to be any advocates for the "shrinking block" interpretation, which is not surprising).

Is there another alternative, as per the 5th option there?

The 5th option is certainly allowed by relativity, otherwise I wouldn't have included it in my article.

Logically speaking, relativity allows you to claim that any set of events you like in 4-d spacetime is fixed and certain, or not. Relativity itself simply does not make any claims either way. Relativity combined with what seems to me like obvious common sense leads to the belief that, at least, all events in our past light cone should be fixed and certain; relativity is what tells us the "past light cone" part (since relativity is what tells us that light propagates with a finite speed) and common sense is what tells us the "fixed and certain" part (because, as I've noted, what happened at past events doesn't change based on what might happen in the future). But if you come across a sufficiently perverse person who insists that some other set of events is fixed and certain and the events in the past light cone aren't, there's no way you can refute him in a logical sense just based on relativity alone.

Would this mean that we can rule out presentism?

No. See above.

Note that when I say there is no global concept of "now" in relativity, that is only talking about the math and the physics of relativity. It does not in any way prevent someone from adding on some other concept of "now" in a philosophical or metaphysical interpretation, that is not contained in the math and the physics of relativity, as long as it is not inconsistent with the math or the physics. Adding on a presentist concept of "now" is not inconsistent with the math or the physics of relativity; it's just not contained in the math or the physics and doesn't change any of the math or the physics.

I had just come to associate the relativity of simultaneity as precluding the possibility of absolute simultaneity

It doesn't, because the term "simultaneity" is referring to two different things in the two terms (which means that the choice of the word "simultaneity" in at least one of them is a bad choice of words).

In "relativity of simultaneity", the word "simultaneity" refers to the notion of simultaneity given by a particular choice of reference frame.

In "absolute simultaneity", the word "simultaneity" refers to some other notion of "simultaneity" that has nothing to do with any choice of reference frame (and also nothing to do with the math or physics of relativity, as above) but comes from someone's chosen philosophical or metaphysical interpretation.

There is no logical connection at all between these two notions.

 

[Nugatory]

I hope you don't take umbridge with me linking you to my reply to Peter, just to save typing the whole thing again.

I'm not going to take umbrage, but I am unclear on how your reply to Peter is relevant to the correctness or incorrectness of your statement about possible relationships between events.

 

[Lynch101]

Yes, all of those possibilities are allowed by relativity (though, as you note, there don't seem to be any advocates for the "shrinking block" interpretation, which is not surprising).

Ah, glad I was able to get that far anyway, thank you!

The 5th option is certainly allowed by relativity, otherwise I wouldn't have included it in my article.

Logically speaking, relativity allows you to claim that any set of events you like in 4-d spacetime is fixed and certain, or not. Relativity itself simply does not make any claims either way. Relativity combined with what seems to me like obvious common sense leads to the belief that, at least, all events in our past light cone should be fixed and certain; relativity is what tells us the "past light cone" part (since relativity is what tells us that light propagates with a finite speed) and common sense is what tells us the "fixed and certain" part (because, as I've noted, what happened at past events doesn't change based on what might happen in the future). But if you come across a sufficiently perverse person who insists that some other set of events is fixed and certain and the events in the past light cone aren't, there's no way you can refute him in a logical sense just based on relativity alone.

I'm not entirely clear on what that 5th option is because I'm not sure in what sense you are using the term "fixed and certain".

In a presentist intepretation it says that events in the past light cone are fixed and certain in the sense that they are over and cannot be changed. A presentist universe comprises only present events. While the other interpretations would be universes which comprise [at least] present and past events.

Note that when I say there is no global concept of "now" in relativity, that is only talking about the math and the physics of relativity. It does not in any way prevent someone from adding on some other concept of "now" in a philosophical or metaphysical interpretation, that is not contained in the math and the physics of relativity, as long as it is not inconsistent with the math or the physics. Adding on a presentist concept of "now" is not inconsistent with the math or the physics of relativity; it's just not contained in the math or the physics and doesn't change any of the math or the physics.

Ah, I see. There is no preferred "now" in the mathematics. I think this is part of the justification that people offer for the Block Universe isn't it? There is nothing which singles out the events on an objects world line, so all events must have equal status. There is nothing to single out events in our past light cone.

It doesn't, because the term "simultaneity" is referring to two different things in the two terms (which means that the choice of the word "simultaneity" in at least one of them is a bad choice of words).

In "relativity of simultaneity", the word "simultaneity" refers to the notion of simultaneity given by a particular choice of reference frame.

In "absolute simultaneity", the word "simultaneity" refers to some other notion of "simultaneity" that has nothing to do with any choice of reference frame (and also nothing to do with the math or physics of relativity, as above) but comes from someone's chosen philosophical or metaphysical interpretation.

There is no logical connection at all between these two notions.

I see. I was thinking that it must be a matter of definition, or the lack thereof.

 

[Lynch101]

I'm not going to take umbrage, but I am unclear on how your reply to Peter is relevant to the correctness or incorrectness of your statement about possible relationships between events.

I was trying to outline the rationale that was given me as to why relativity necessitates a block universe. I was hoping that the errors in reasoning or mistaken assumptions might be explained to me because I cannot identify them myself.

Peter mentioned, in a proceeding post, the point that there is nothing in the mathematics of relativity which picks out the present moment - I presume this is neither globally nor locally. This is another argument I have heard in favour of the Block Universe, that no events are singled out over any other events in the mathematics, so all events must share the same status.

If a universe is said to comprise present events, then it must also be said to comprise past and future events, on the basis that nothing in the mathematics favours present events over the others.

 

[PeterDonis]

I'm not entirely clear on what that 5th option is because I'm not sure in what sense you are using the term "fixed and certain".

I'm not sure what isn't clear about what I explained before.

In a presentist intepretation it says that events in the past light cone are fixed and certain in the sense that they are over and cannot be changed.

I think you need to give a source for this statement. (I think you need to give sources more generally for any interpretation you want to discuss at this point, since it seems evident that your own personal understanding of every interpretation you've mentioned is flawed. We need to be looking at what the actual proponents of each of these interpretations say, not what you think they say.)

The entry on "presentism" in the Stanford Encyclopedia of Philosophy [1] says

"Presentism is the view that only present things exist."

There is no mention anywhere in that article (even in the section that specifically discusses relativity and its relationship to presentism) of presentism claiming that events in the past light cone are fixed and certain and cannot be changed.

[1] https://plato.stanford.edu/entries/presentism/

A presentist universe comprises only present events.

That is consistent with the quote above from the Stanford Encyclopedia of Philosophy, yes.

There is no preferred "now" in the mathematics.

Yes.

think this is part of the justification that people offer for the Block Universe isn't it?

Sort of. You are conflating a number of very different statements. See below.

There is nothing which singles out the events on an objects world line, so all events must have equal status.

This is a non sequitur. Also "there is nothing which singles out the events on an object's worldline" is a different statement from "there is nothing which singles out a preferred now".

Not only that, but "there is nothing which singles out the events on an object's worldline" is false; the fact of the events being on the object's worldline singles them out.

There is nothing to single out events in our past light cone.

Yes, there is. Light cones are invariant features of the spacetime geometry. There are light cones in the math, whereas there is no preferred "now" in the math.

Further, "events in our past light cone" is also different from "events in some spacelike 3-surface including our present event", which is the kind of "now" of which there is no preferred "now" in the math.

I get the strong sense that you do not have a good grasp of the basic mathematics of relativity. If you don't have that, trying to make sense of the various possible interpretations of relativity is not going to work out well. Have you studied a basic textbook on relativity, such as Taylor & Wheeler's Spacetime Physics? Or Chapter 1 of Carroll's online lecture notes on relativity [2]?

[2] https://arxiv.org/abs/gr-qc/9712019

 

[Lynch101]

I'm not sure what isn't clear about what I explained before.

My apologies, I'm still not certain. I'm not sure in what sense you are using the terms fixed and certain. Is it in the sense that the universe comprises past events or in the sense that the universe comprises only present events, but past events are fixed and certain because they are finished with? Or, is there another sense? Is it that the Universe comprises past events in our past light cone as well as our present? If it is the latter sense, then I am unclear as to whose past light cone events the Universe comprises.

I think you need to give a source for this statement. (I think you need to give sources more generally for any interpretation you want to discuss at this point, since it seems evident that your own personal understanding of every interpretation you've mentioned is flawed. We need to be looking at what the actual proponents of each of these interpretations say, not what you think they say.)

The entry on "presentism" in the Stanford Encyclopedia of Philosophy [1] says

"Presentism is the view that only present things exist."

There is no mention anywhere in that article (even in the section that specifically discusses relativity and its relationship to presentism) of presentism claiming that events in the past light cone are fixed and certain and cannot be changed.

[1] https://plato.stanford.edu/entries/presentism/
...
That is consistent with the quote above from the Stanford Encyclopedia of Philosophy, yes.

I am trying to apply the terminology that you have been using to a presentist Universe, given the difficulty with terms such as "exist". You don't seem to have completely ruled out the idea that presentism is compatible with the idea of fixed and present, but I don't think that you've expressly stated that it is either.

If we apply the terms fixed and certain to a presenist universe (be that Newtonian or otherwise), then it has a different meaning than when it is applied to a Universe which comprises past and present events.

This is a non sequitur. Also "there is nothing which singles out the events on an object's worldline" is a different statement from "there is nothing which singles out a preferred now".
...
Not only that, but "there is nothing which singles out the events on an object's worldline" is false; the fact of the events being on the object's worldline singles them out.

Yes, my apologies, I skipped straight to a related point and possibly didn't articulate it clearly enough.

Is there something in the mathematics which singles out or preferences certain events on an objects world line, over other events on the same world line? I was under the impression that there isn't.

Yes, there is. Light cones are invariant features of the spacetime geometry. There are light cones in the math, whereas there is no preferred "now" in the math.
...
Further, "events in our past light cone" is also different from "events in some spacelike 3-surface including our present event", which is the kind of "now" of which there is no preferred "now" in the math.

There are light cones yes, but is there anything in the math which preferences events in the past light cone over events in the future light cone?

The contention that there isn't is one of the arguments in favour of the Block Universe.OK,

I get the strong sense that you do not have a good grasp of the basic mathematics of relativity. If you don't have that, trying to make sense of the various possible interpretations of relativity is not going to work out well. Have you studied a basic textbook on relativity, such as Taylor & Wheeler's Spacetime Physics? Or Chapter 1 of Carroll's online lecture notes on relativity [2]?

[2] https://arxiv.org/abs/gr-qc/9712019

Thank you, I'm always on the lookout for new resources. I'll check it out. I've tried going through a few textbooks and I can follow the math when I see someone doing it out.

But, a common argument in favour of the Block Universe is that there is nothing in the mathematics which preferences certain events on the world line of an object over other events, be they in the past light cone or in the present. That is correct, isn't it?

 

[Nugatory]

I was trying to outline the rationale that was given me as to why relativity necessitates a block universe. I was hoping that the errors in reasoning or mistaken assumptions might be explained to me because I cannot identify them myself.

I did say "That cannot happen" in response to your flat-out incorrect statement:

It can happen that an event in the past light cone of A ("A considers it to have happened") is in neither the past nor the future light cone of B ("Anything B says about whether it has happened or not depends on B's completely arbitrary choice of convention for assigning time coordinates to events outside the the light cones") or vice versa. And note that even that cannot happen when they're passing one another; when they're both at the same point in space they have the same past and future lightcones.

You might want to try drawing the past and future light cones of the event "A and pass one another" and the worldlines of A and B, see what relationships are possible between events insde and outside of these lightcones.

 

[PeterDonis]

I'm not sure in what sense you are using the terms fixed and certain. Is it in the sense that the universe comprises past events or in the sense that the universe comprises only present events, but past events are fixed and certain because they are finished with?

I have no idea because I don't know what "comprises" means or how to test for it. I do know how to test whether, for example, events in my past light cone are fixed and certain: do they ever change? That is testable because we have records of past events; those records don't change depending on what happens in the future, so the test shows that events in our past light cone are fixed and certain.

So, since I have a way to test "fixed and certain" but don't have a way to test "comprises", trying to define "fixed and certain" in terms of "comprises" seems like a step backwards, in the direction of more vagueness, not less.

I am trying to apply the terminology that you have been using to a presentist Universe

I don't know that you can. Different philosophical and metaphysical interpretations don't always allow the use of the same terminology, at least not with the same meanings of the terms.

You don't seem to have completely ruled out the idea that presentism is compatible with the idea of fixed and present, but I don't think that you've expressly stated that it is either.

I don't know what "fixed and present" means. If you mean "present events are fixed and certain", I think presentism is compatible with that idea; indeed, at least some versions of presentism seem to be that idea.

Is there something in the mathematics which singles out or preferences certain events on an objects world line, over other events on the same world line?

In the bare math, no. But the bare math does not apply to the real world as it is, because in the real world, we are experiencing a particular present event now, and we have to have some way of reflecting that in the math. The way we do that is to pick some event on Earth's worldline and have that event correspond to our present event. Once we do that, that present event is singled out, and events on Earth's worldline are divided into that event, events to its past, and events to its future.

is there anything in the math which preferences events in the past light cone over events in the future light cone?

In the bare math, no, the math is symmetric between past and future. More precisely, the bare math does not tell you which half of the light cone is the "past" half and which half is the "future"half. But again, in order to apply the math to the real world, we have to label one half of the light cone the "past" half and the other half as the "future" half, since in the real world the future is not the same as the past; we remember the past but cannot remember the future, for example. And once we've made that choice for the light cones at one event, any event, it must be the same at every event--that part is required by the math of Minkowski spacetime.

The contention that there isn't is one of the arguments in favour of the Block Universe.

Please give a reference for this claim. My previous remarks about this apply here.

a common argument in favour of the Block Universe is that there is nothing in the mathematics which preferences certain events on the world line of an object over other events, be they in the past light cone or in the present

Again, please give a reference for this claim. We need to see the actual sources for these arguments you are referring to.

 

[Dale]

If you mean "present events are fixed and certain", I think presentism is compatible with that idea; indeed, at least some versions of presentism seem to be that idea.

I think that the problem with “fixed and certain” in this context is that it is closely tied to determinism. What would be “fixed and certain” would vary depending on whether or not the laws of physics are deterministic, regardless of what is “real”.

 

[PeterDonis]

What would be “fixed and certain” would vary depending on whether or not the laws of physics are deterministic

If we extend things beyond our past light cone, yes. To me that's one of the advantages of the alternative view I proposed, that it only claims events are fixed and certain in the region of spacetime where our common experience already tells us they are anyway, so it doesn't require any commitment either way on the determinism question.

 

[Lynch101]

I did say "That cannot happen" in response to your flat-out incorrect statement:
You might want to try drawing the past and future light cones of the event "A and pass one another" and the worldlines of A and B, see what relationships are possible between events insde and outside of these lightcones.

I don't think I did the best job of explaining the point. I thought you guys might recognize the point I was trying to make, but I will reference a more reliable source than my memory to try and clarify it. The causal relation between the events is important because of what we can deduce at a later time.

The example given below makes perfect sense to me, but I know that the use of the term "real" here might cause some problems - and there may be a hidden assumption that I am not aware of. I understand the point the author is trying to make below but I wasn't necessarily able to define the term "real", I just understood the point he was trying to make. I have been giving it some thought and I'm wondering if the follwoing makes sense:
I think the below might make sense if we think of "real events" as those, not as events which have had or which will have a causal influence, but as events which can have a causal influence. The opposite of this, unreal events, cannot have a causal influence because unreal events do not happen. I think this helps to bring the term "real" into the domain of observable consequences and hopefully helps to make sense of the below. It also does not contradict the intuitive understanding of the term, that most people [seem to] have.

The general idea is this: Events, which exist now, must be regarded as real. Saint Augustine already expressed our ordinary beliefs that past and future events do not possesses reality. The Special theory of relativity has, however, introduced a complication into this view. The Now, even if we deny that it is purely subjective, has become frame-dependent. So we must introduce the coordinate system of observer O, who will regard all events as real, which lie on O’s space-like simultaneity planes. These are all the events simultaneous with O’s Now. But what is now for one observer will not be now for another. There will be a second observer O , in relative constant motion with respect to O (Fig. 4.16). If the observers are space-like separated from the event, then the time order will not be unique for them.

The second observer’s reference frame can be made coincident with that of O, by a convenient choice. That means that both coordinate systems can be made to coincide at the origin. Both observers will be in each other’s present. There will be many events, which are simultaneous and hence real for one observer, which lie in the past or future for a second observer. Take an event E1, which is already past for one observer but still in the future of the other observer. This event cannot be determinate for one and indeterminate for the other observer. It must be determinate for both observers. For what is real for one observer must be real for another observer in his presence. Take an event E2, which is now for one observer but lies in the future of the other observer. Then it is real and determinate for one observer; so it must be real and determinate for the other observer. Hence all events must already be determinate and equally real for all observers.

Consider this argument in more technical detail. The space-time representation of the universe is given in Fig. 4.16. It will always allow us to find a distant observer, O1 for whom an event, which is now or future for some other observer O2, will already exist in his past. O1 is in relative constant motion with respect to O2. They synchronize their clocks at time t such that t1 = t2 = 0. Then O1 will consider certain events to be simultaneous: those events, which lie on a space-like hyperplane, x1, which lies at an angle α to O1’s time axis. Because O2 is stationary with respect to O1, observer O2 will regard different events as simultaneous, since for this observer the space-like simultaneity plane, x2, forms an angle of 90◦ with respect to O2’s time axis. Then there will be an event, E, which will belong to the past for observer O1, lying below O1’s simultaneity plane. The same event, E, will however reside in the future of observer O2, lying above O2’s simultaneity plane.

This has a puzzling consequence for the ontological status of E. If E is determinate for observer O1, since it lies in O1’s past, how can E be indeterminate for observer O2, even though it lies in O2’s future? It seems that the proponents of the block universe provide the only sensible answer. All events must be regarded as determinate and real at all times. O2 cannot influence event E, although it lies in this observer’s future. According to the Special theory of relativity, there is always an observer like O1 for whom each future event in O2’s frame of reference is already a past event. Our ordinary conception must be mistaken. If an event is already determinate for one observer, it must be determinate for all observers. Equally, if an event is already real for one observer, it must be real for all observers. The physical world is a block universe. The passage of time is a human illusion.

In discussions I've had and discussions I've read, the objection to this usually relies on the idea that co-ordinate reference frames, and therefore the ordering of events, are arbitrary. The reply to this has been that an arbitrary choice of co-ordinate frames doesn't affect an events ability to have a causal influence on other events. An egg dropping on the floor will break, regardless of the co-ordinate system used to describe it. In the case of event E above, at a moment that O2 considers to be in the future, E can have a causal influence on other events. That it lies outside the light cone of O1 and O2 just means that it hasn't yet had a causal influence on them.Hopefully that makes a bit more sense than the way I was trying to explain it.

 

[Lynch101]

I have no idea because I don't know what "comprises" means or how to test for it. I do know how to test whether, for example, events in my past light cone are fixed and certain: do they ever change? That is testable because we have records of past events; those records don't change depending on what happens in the future, so the test shows that events in our past light cone are fixed and certain.

So, since I have a way to test "fixed and certain" but don't have a way to test "comprises", trying to define "fixed and certain" in terms of "comprises" seems like a step backwards, in the direction of more vagueness, not less.

Are you familiar with the term comprise in a general context? You are obviously familiar with ideas such as the Block Universe, the growing block, and presentism. Would you agree that they offer different picutres of the Universe or the structure of the Universe?

Considering the Block Universe juxtaposed with presentism, or a presentist interpretation of Newtonian physics: in your understanding of these ideas, would you say that "fixed and certain" has different connotations in each?

Testing
Wit regard to testing, we cannot test events in our past light cone, we can only make observations of the present. The records we make of events are not the events themsevles, they are separate events in and of themselves. So, while our records of events might not change, it is a non-sequitir to say that the events, to which those records correspond have not changed. Those records continue to persist in the present, the "now".

An example would be a photo of your 10th birhtday. That photo remains in the present. It is not a test of past events. Those records don't change depending on what happens in the future, but does the future change because of what happens in the past? We can't test that.

"Real"
I was thinking more about that term, "real". I was reading a few sources trying to find references for this discussion and I came across it again, and again. I wasn't sure why, but I felt I understood what the authors were trying to get at, even though they never defined the term real, and neither could I. That forced me to think about it more.

I think we can think of "real events" as those events which can have a causal influence on other events/objects. Events which are in our past light cone have had a causal influence, but this leaves out those events which have not yet had a causal influence. Of course, we cannot know what those events are until light from them reaches us, but when it does, we can backwards rationalise that prior to the light reaching us, that event must have been able to have a causal influence on other events, closer to it. Events in this "elsewhere" region can have causal influence on other events, even if we only find out what those events were at a later point.

I don't know that you can. Different philosophical and metaphysical interpretations don't always allow the use of the same terminology, at least not with the same meanings of the terms.

I think we can apply the notion of an event that can have causal influence to all cases also. Would it have the benefit of including those events which lie outside our past light cone?

I don't know what "fixed and present" means. If you mean "present events are fixed and certain", I think presentism is compatible with that idea; indeed, at least some versions of presentism seem to be that idea.

My apologies, I mis-typed. I meant "fixed and present".

Would you agree that "fixed and present" has different connotations when applied to the growing block universe and a presentist universe?

In the bare math, no. But the bare math does not apply to the real world as it is, because in the real world, we are experiencing a particular present event now, and we have to have some way of reflecting that in the math. The way we do that is to pick some event on Earth's worldline and have that event correspond to our present event. Once we do that, that present event is singled out, and events on Earth's worldline are divided into that event, events to its past, and events to its future.

In the bare math, no, the math is symmetric between past and future. More precisely, the bare math does not tell you which half of the light cone is the "past" half and which half is the "future"half. But again, in order to apply the math to the real world, we have to label one half of the light cone the "past" half and the other half as the "future" half, since in the real world the future is not the same as the past; we remember the past but cannot remember the future, for example. And once we've made that choice for the light cones at one event, any event, it must be the same at every event--that part is required by the math of Minkowski spacetime.

Please give a reference for this claim. My previous remarks about this apply here.

Again, please give a reference for this claim. We need to see the actual sources for these arguments you are referring to.

Is the validity of the argument dependent on who has said it before?

I understand that you probably just want a more rigorous statement of the point, instead of my attempt to describe it. Unfortunately I haven't kept notes on the vast amount of reading and watching I've done on the subject. I never thought I'd be in a position where I would be trying to present the case for the Block Universe . I think you might be familiar with the argument in a different guise. It is the argument that all events on an objects world line have to be considered equally "real" because the math doesn't indicate a preference for any single event over any other.

I did do a bit of a trawl last night to see if I could come across any references for you, though. It appears as though the author is considering some hybrid models, in a similar vein to yours. I still haven't fully understood the model you are suggesting so I'm not sure if it is fully covered by the author.

Each of the three sorts of region (a past light cone, a future light cone, and a bowtie) are regions definable within Minkowski spacetime, once point p is chosen. None of these hybrids resurrects absolute simultaneity. Of course, the eternalist will deny that any point p within Minkowski spacetime is distinguished in the way required by these Hybrids.

The last sentence here is the point I was referring to.

He goes on to provide additional context about privileging:

A final critical problem is that each of Hybrids 3 to 5 grants a special privilege to a single point in reality. On each view there exists a ‘generator point’, a point p such that all other points in reality are: (1) in the absolute past of p (Hybrid 3); (2) in the absolute future of p (Hybrid 4); or (3) spacelike separated from p (Hybrid 5). The rest of the points in spacetime are equally part of reality, but are not generator points.

Notice that in Broad's (non-relativistic) growing-block universe there is a privileged class of points. The points on the crest of the wave generate reality (in the sense that spacetime consists of the class of points before the members of this class), whereas other points in spacetime are not generators in this sense. This inegalitarianism is related to the (not unintuitive) conviction on the part of the view's defenders that time and space are importantly disanalogous. While it would be implausible to say that one region of space is ontologically privileged, it is not implausible, Broad could claim, to say that one region of time is ontologically privileged. This defense of inegalitarianism could perhaps be extended to Hybrids 3 and 4, since the generator point in each case enjoys a purely temporal distinction: it is time-like related to all other points. But the defense utterly fails for Hybrid 5, the bowtie view, since on that view the generator point is simultaneous, relative to suitably chosen frames of reference, with other points in reality. This asymmetry is important. As noted above, Hybrids 3 and 4 do not retain much of the original spirit of presentism, since reality contains dinosaurs on Hybrid 3 and Martian outposts on Hybrid 4, but Hybrid 5 seems closer to the original spirit of presentism. The additional argument against it is therefore welcome: Hybrid 5 implies the existence of an implausibly distinguished point. Located at the generator point, I could say truly that reality consists of all points with spacelike separation from me. An utterance by you, located across the room, would be wrong.

He goes on to say:

There is an alternative theory, the B-theory, which is consistent ‘as-is’ with contemporary science and suffers
no apparent philosophical defects. At the least, tentative rejection of presentism seems in order.
The four-dimensional picture is that of a world spread out in time populated by spacetime worms, sums of
instantaneous stages from different times.

Here he is referring to the Block Universe and the apparent lack of philosophical defects includes the previously discussed idea of privileging events.

 

[PeterDonis]

The example given below makes perfect sense to me

It's the same argument that I refute in the Insights article. I don't see the point of refuting it again here.

an arbitrary choice of co-ordinate frames doesn't affect an events ability to have a causal influence on other events

That's correct: the causal relationship between events, i.e., whether they are timelike, null, or spacelike separated, is invariant and independent of any choice of coordinates.

In the case of event E above, at a moment that O2 considers to be in the future, E can have a causal influence on other events.

This, however, is wrong. Events are points in spacetime. The causal relationship between E and any other point in spacetime is what it is; it can't change, because "where" in spacetime a particular point is can't change.

What would be true is to say that, while E cannot have a causal influence on an event spacelike separated from it, say some event A, it can have a causal influence on events in the future light cone of A, some of which might be on the worldline of some object whose worldline also passes through event A. I suspect this sort of thing is what you are actually thinking of.

Are you familiar with the term comprise in a general context?

Yes. But we're not in a general context here. We're in a specific context in which a claim is not scientific unless there is a way to test it. There's no way to test what events "comprise" the Universe that I can see.

Considering the Block Universe juxtaposed with presentism, or a presentist interpretation of Newtonian physics: in your understanding of these ideas, would you say that "fixed and certain" has different connotations in each?

No.

we cannot test events in our past light cone, we can only make observations of the present.

Wrong. Our observations are of information reaching us at our present event, but that information comes from events in our past light cone, and only those events--it is impossible for us to get information at our present event that is outside the past light cone of that event, because information cannot travel faster than light.

Those records don't change depending on what happens in the future

Not only that, but you can in principle continue to get new information about the same past events. For example, you find out that someone else also took a photo at your 10th birthday party, and what is in their photo is consistent with what is in your photo. That is a fresh test of the fact that past events are fixed and certain.

does the future change because of what happens in the past? We can't test that.

Yes, that's correct, because we can't re-run the future again with a different past.

I think we can think of "real events" as those events which can have a causal influence on other events/objects.

How would you test this? The only way to test it is to observe such an influence, and the only way to do that is for all the events in question to be in your past light cone. So any event for which you have evidence of it being "real" in this sense must be in your past light cone, and hence is fixed and certain according to my proposed view.

I think we can apply the notion of an event that can have causal influence to all cases also. Would it have the benefit of including those events which lie outside our past light cone?

No. See above.

I meant "fixed and present".

That's what you typed, and that's what I read. So I don't know what you think you have changed here.

Would you agree that "fixed and present" has different connotations when applied to the growing block universe and a presentist universe?

My response has not changed, since "fixed and present", as above, is what you typed before, and what I responded to.

Is the validity of the argument dependent on who has said it before?

No, but what the argument actually is is best assessed by reading the argument as it is presented by the person who originally made it. That is not you. I don't want to read what you think someone else's argument says. I want to read what they say.

A good example is the quote you gave from Friedel Weinter, which, now that I read what that person actually said, turns out to be the same argument I already refuted in the Insights article. If I had only your description to go on, I would not have known that and would not have been able to save time by stating that I've already refuted that argument once and don't see the point of doing it again.

I think you might be familiar with the argument in a different guise.

The argument I refuted in my Insights article is the argument I am familiar with. So far, the only actual reference to an argument that you have given (the Friedel Weinter quote you gave) is making that same argument, as I noted above. I don't see the point of refuting the same argument over and over; the whole reason I wrote the Insights article was to not have to do that.

He goes on to provide additional context about privileging

This context is pretty useless without knowing what the different views he refers to actually say.

However, I think there is a general point that can be made. It seems to me that this author is implicitly struggling with an idea that he doesn't state explicitly: the idea that what is "real" depends on which event in spacetime is your present event. That is what he seems to be objecting to when he talks about "privileged" points in spacetime. But of course which point is "privileged" in this sense is just whichever point is our present event--our "here and now". And this event changes as we experience our lives. So there is no point that is always privileged; there is just the obvious fact that which event in spacetime is our "here and now" is not always the same. Every point on our worldline gets its chance to be "privileged", and other points on other observers' worldlines get their chance to be "privileged" as well.

I discussed this point in the Insights article. Relativity does not require that there has to be any criterion for what is "real" that is the same for all events in spacetime. It is perfectly consistent with the view that what is "real" for us, here and now, might be different from what is "real" for us tomorrow, or what is "real" for observers somewhere in the Andromeda galaxy at an event spacelike separated from us here and now. This, of course, is an obvious implication (if we treat "real" as synonymous with "fixed and certain") of my proposed view that only events in the past light cone of our present event are fixed and certain.

 

[Lynch101]

It's the same argument that I refute in the Insights article. I don't see the point of refuting it again here.

A good example is the quote you gave from Friedel Weinter, which, now that I read what that person actually said, turns out to be the same argument I already refuted in the Insights article. If I had only your description to go on, I would not have known that and would not have been able to save time by stating that I've already refuted that argument once and don't see the point of doing it again.
...
No, but what the argument actually is is best assessed by reading the argument as it is presented by the person who originally made it. That is not you. I don't want to read what you think someone else's argument says. I want to read what they say.

Thank you Peter, I appreciate your taking the time thus far. I understand that it must be frustrating to be going over the same ground again. The purpose of this thread is, essentially, to understand your article better.

My understanding of your argument is that you say BU proponents "help themselves" to the second premise of the argument, that all events are "equally real". That is similar to a point I had raised myself, when first learning about relativity, but the counter argument that was put to me is that proponents don't just "help themselves" to this part of the premise, this part of the premise derives from the fact that the mathematics do not privilege any events on the world lines of objects, over any other events. This is demonstrated by the quote from Theodore Sider.

That part of the premise is arrived at by taking the mathematics at face value and not adding any additional assumptions or intuitions. If that is true, as you seem to have confirmed yourself, then I don't think that the contention stands - that the BU helps itself to the second premise.

That's correct: the causal relationship between events, i.e., whether they are timelike, null, or spacelike separated, is invariant and independent of any choice of coordinates.

This, however, is wrong. Events are points in spacetime. The causal relationship between E and any other point in spacetime is what it is; it can't change, because "where" in spacetime a particular point is can't change.

I might not have elucidated that point clearly enough. I'm not suggesting that the causal relationship between E and any other point in spacetime can change, its simply a statement about E's ability to have a causal influence on other, unspecified events. If it cannot have a causal influence on other events, then it cannot happen and will never be observed.

Of course, we can only work out after the fact when an event happened, and started having a causal influence, but that allows us to fill in the blanks in our pictures of previous "nows". Take the example of Albert standing on the platform, midway between two light sources, which happen simultaneously in his frame. He can only know that the events happened simultaneously after the light has reached him. However, once the light has reached him, he can calculate that those events must have happened at a moment prior to when the light reached him, given the finite speed of light. Here therefore knows that these events were having a causal influence on other events "elsewhere" as the light made its way towards him. He concludes that in his frame, the events happened at the precise moment when Hendrick was co-located with him (or when he passed him at the midway point). Studying is spacetime diagrams, he can see that, according to Hendrick's frame, the event/light was having a causal influence on other events prior to the moment when they crossed paths. It must have been having such a casual influence because otherwise, it would not have arrived to Hendrick when it did, or it wouldn't have arrived to either of them.In the example above, if we say that t1=t2=0 and we also say that t1=t2=0= O1's 30th birhtday and O2's 30th birhtday. At t=0 event E is having a causal influence on events "eleswhere" in the universe. Neither O1 nor O2 can know this at t=0, but after the fact, they can do their calculations and draw their spacetime diagrams and analyse the information.

Both can see that at t=0, event E was already having a causal influence on other events before O1 and O2 crossed paths. How can an event have a casual influence if it hasn't happened yet? How can it have happened prior to the moment when O1 and O2 cross paths, according to O1, yet not yet have happened at exactly that moment according to O2, where O2 would say that it was in her future? The co-ordinate reference frames might be arbitrary but the causal influence of an event is not dependent on the choice of co-ordinates.

If we think in terms of 30th birhtday's, at t=0 both turned 30. O1 says event E happened before their joint 30th birthdays. That is, O1 says that the event was already having a causal influence on other events in the Universe prior to their 30th birhtday encounter. O1 says that event E was having a causal influence on other events at a moment when O2 says the event hadn't happened yet. If O1's calcuations are correct, how can an event be influencing other events cuausally, if it hasn't yet happened? The Block Universe would appear to be the only reasonable solution to this, from the arguments I've heard.

What would be true is to say that, while E cannot have a causal influence on an event spacelike separated from it, say some event A, it can have a causal influence on events in the future light cone of A, some of which might be on the worldline of some object whose worldline also passes through event A. I suspect this sort of thing is what you are actually thinking of.

Yes, but that is not all we can say, as outlined above.

Yes. But we're not in a general context here. We're in a specific context in which a claim is not scientific unless there is a way to test it. There's no way to test what events "comprise" the Universe that I can see.

I didn't find this answer very satisfying when I first heard it myself, but I found it hard to dispute and ultimately convincing of the case for the Block Universe. Maybe you will be able to help me: the criteria which you apply for testing events - events in the past light cone - applies equally in a Block Universe. At every point along your world line the you that corresponds to that moment has observed all the events in the past light cone. We do not have the knowledge of future events because of our location in spacetime, but there is a future version of ourselves for whom those events are in the past light cone, and so your own criteria would be fulfilled.

We arrive at this conclusion, not by assuming it, but by taking the mathematics at face value and not adding any assumptions or intuitions.

No.

The block and growing block unvierses would say that events are fixed and certain because they are in our past light cone and the universe comprises them (in the general meaning of the word), or they persist in the overall structure of the universe - this is what gives it a block strucutre.

Contrast this with a presentist universe, which doesn't have such a block structure. In a presentist universe events are fixed and certain because they are over. However, the Universe does not comprise (in the general meaning of the word) past events. They do not persist in the structure of the Universe, hence why there is no block structure.

It might help to think of a presentist universe as one which is extended in only 3 spatial dimensions, but not in a temporal dimension (its more of a single temporal point), while a 4D block universe is extended in 3 spatial dimensions as well as extended temporally. In these different contexts "fixed and certain" has different connotations.

Wrong. Our observations are of information reaching us at our present event, but that information comes from events in our past light cone, and only those events--it is impossible for us to get information at our present event that is outside the past light cone of that event, because information cannot travel faster than light.

Not only that, but you can in principle continue to get new information about the same past events. For example, you find out that someone else also took a photo at your 10th birthday party, and what is in their photo is consistent with what is in your photo. That is a fresh test of the fact that past events are fixed and certain.

Thank you, I should have been more precise.

We cannot test events which are in our past, which are over according to us. We cannot test our 10th birthday to see if it has changed. To do so would require time travel. The photo of our 10th birthday is a record that was taken at the time, but they are separate events in and of themselves i.e. the records are not the events in themselves. We can see this by virtue of the fact that the records persist in the present. We can test the photo to see if it has changed, but we cannot test the past event.

The corroborating photo simply shows that, during our 10th birthday other observers observed the same thing, but it doesn't tell us about the state of past events. If we burn the photos, it has no bearing on the past events because they are separate. Now, that is not to say that the past does change, its simply that we cannot test it.

We can only carry out tests in the present on light which reaches us from events which happened in the past. This is part of the reason how, in the example given above, O1 and O2 can calculate when event E had to start having a causal influence in the Universe in order to arrive at O1 when it does. They conclude that, it must have started having this causal influence prior to t=0. However, at t=0, event E was in O2's future. So, O2 would have to agree - on the basis of knowing what relativity shows - that an event in their future was having a causal influence in the universe. O2 would have to conclude that, at t=0, the Universe comprises (in a general sense) events which O2 considered to be in the future. If it didn't, then it wouldn't have the causal influence on O1 that it ends up having.

This conclusion is supported by the fact that the mathematics doesn't privilege any particular events on an objects world line over any other events i.e. by taking the math at face value and not adding any additional assumptions or intuitions.

How would you test this? The only way to test it is to observe such an influence, and the only way to do that is for all the events in question to be in your past light cone. So any event for which you have evidence of it being "real" in this sense must be in your past light cone, and hence is fixed and certain according to my proposed view.

Indeed, all of the evidence points to its validity. If an event which doesn't happen is demonstrated to have a causal influence, the the hypothesis would be invalidated.

The above doesn't contradict the Block Universe however, as outlined above. If no moment along an observers world line is more special than any of the others, as the mathematics when we don't append anything to them, then at every moment that observer observes what is in their past light cone. This is true for every moment along the world line, up until the observers death, and beyond (given that their corpse persists). Each moment can only observe the past light cone for that moment, but if no moment is singled out over any other, then all events are observed up until the death of the observer.

Again, this isn't assumed it's what the mathematics seems to imply.
No. See above.

That's what you typed, and that's what I read. So I don't know what you think you have changed here.
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My response has not changed, since "fixed and present", as above, is what you typed before, and what I responded to.

My apologies, I did it twice!

I meant "fixed and certain"

No, but what the argument actually is is best assessed by reading the argument as it is presented by the person who originally made it. That is not you. I don't want to read what you think someone else's argument says. I want to read what they say.

OK, so there are two arguments. One w

The argument I refuted in my Insights article is the argument I am familiar with. So far, the only actual reference to an argument that you have given (the Friedel Weinter quote you gave) is making that same argument, as I noted above. I don't see the point of refuting the same argument over and over; the whole reason I wrote the Insights article was to not have to do that.

There are two references there, one from Friedel Weiner and the other form Theodore Sider in this post (at the bottom). The argument from Sider is a refutation of your contention that Block Universe proponents "help themselves" to the second premise. His is an example of the point I was making, for which you asked a reference on; that the second premise is assumed. Sider points out the fact, which you reiterated, that the mathematics does not single out any event(s) over any others on the world line of an object. It is on this basis that BU proponents base the statement that all events on an objects world line are "equally real".

The argument from Weiner is separate, but related. I'm not sure your article actually refutes that point either because it seems to focus on the contention that the conclusion is assumed. You mention the Andromeda paradox, but only in the context of assuming the conclusion. The argument from Sider speaks to this point and, I believe, demonstrates that the premise isn't assumed.

The argument from Weiner would still require an answer, however, as to how an event which you say hasn't happened yet can have a causal influence in the Universe - not on you, but on other objects in the Universe - as it must do, in order for the observer moving relatively to you, to observe it when they do.

This context is pretty useless without knowing what the different views he refers to actually say.

Not for the point that is being made. The point being made is that BU proponents don't assume the premise, it is based on the fact that the math doesn't privilege any events on an objects world line over any others. You asked for a reference for this particular point, to which Sider is that reference.

The context he provides says that other hybrid models do privilege specific events. If you are interested in those other models, I have referenced the name of the book there and the pages. But, that isn't necessary for the point that the math doesn't privilege any events over any others, this just offers a juxtaposition for added content. He also makes the explicit statement on the point, which preceded it and which emboldened. The latter part was just for additional context.

However, I think there is a general point that can be made. It seems to me that this author is implicitly struggling with an idea that he doesn't state explicitly: the idea that what is "real" depends on which event in spacetime is your present event. That is what he seems to be objecting to when he talks about "privileged" points in spacetime. But of course which point is "privileged" in this sense is just whichever point is our present event--our "here and now". And this event changes as we experience our lives. So there is no point that is always privileged; there is just the obvious fact that which event in spacetime is our "here and now" is not always the same. Every point on our worldline gets its chance to be "privileged", and other points on other observers' worldlines get their chance to be "privileged" as well.

What would lead you to conclude that the author is implicitly struggling with that?

Indeed, you might be correct, but he still points out that the reason for not privileging any event over any other is because the mathematics doesn't stipulate this. Therefore, to designate any event as the "real" present, for any reason whatsoever, is to add an additional assumption that isn't implicit in the mathematics. As he says, there is an interpretation which doesn't require this assumption, and that is the Block Universe. So, I don't think the contention of your article stands.

In addition, none of the above contradicts the Block Universe interpretation.

I discussed this point in the Insights article. Relativity does not require that there has to be any criterion for what is "real" that is the same for all events in spacetime. It is perfectly consistent with the view that what is "real" for us, here and now, might be different from what is "real" for us tomorrow, or what is "real" for observers somewhere in the Andromeda galaxy at an event spacelike separated from us here and now. This, of course, is an obvious implication (if we treat "real" as synonymous with "fixed and certain") of my proposed view that only events in the past light cone of our present event are fixed and certain.

It is precisely that relativity does not set out any criteria for what is "real" that all events should be considered equally "real". There is nothing in the mathematics to distinguish a "real" present or a "here and now" form any other event on the world line.

The Block Universe is perfectly consistent with the points you raise above, it just doesn't add anything to the math to designate something as "real for us" or "here and now" because the math doesn't set out the criteria for any such ideas. As you have said, the "fixed and certain" criteria you set out is perfectly consistent with the Block Universe.